Determinação de enxofre em petróleo pesado resíduo de vácuo e coque por ICP OES após combustão iniciada por microondas em sistema fechado

Abstract

In this work was proposed a microwave induced combustion procedure in closed vessels for decomposition of heavy crude oil, vacuum residue and coke for further sulfur determination by inductively coupled plasma optical emission spectrometry (ICP OES). For procedure, a small disc of filter paper (12 mg) is placed together with the sample pellet on a quartz holder and the sample holder is placed inside of a commercial quartz vessel commonly used for conventional microwave assisted wet digestion. After ammonium nitrate solution (50 μL of 6 mol L-1) is added to the paper as aid for ignition. The vessel is charged with oxygen (20 bar) and sample combustion is started by microwave radiation about 10 s after the microwave radiation is applied. The kind of absorbing solution was studied (H2O, H2O2, (NH4)2CO3 and HNO3) at different concentrations and the necessity of using additional reflux step after combustion for complete analyte recovery. Quantitative recoveries were obtained by using 0.05 mol L-1 (NH4)2CO3 or 5% (v/v) H2O2 solutions for combustion with 5 min of reflux. The results obtained by MIC were close to those obtained by microwave assisted wet digestion. The concentration of sulfur in samples decomposed by MIC was also determined by ion chromatography (IC) and the obtained results were similar to those obtained by ICP OES showing that MIC is suitable for subsequent sulfur determination by both techniques. The MIC procedure allows the decomposition of sample mass up to 500 mg with residual carbon content (RCC) of digests lower than 1%. The agreements for decomposition of certified reference material by MIC were 99.1 ± 1.1% and 100.1 ± 1.2%, respectively, with the use of 5% (v/v) H2O2 solution and 0.05 mol L-1 (NH4)2CO3 solution, for combustion with application of reflux step. The proposed MIC procedure allows the decomposition in less time that conventional wet digestion without using concentrated acids. The procedure shows operational safety and eight samples can be simultaneously decomposed. Moreover, the decomposition in closed system minimizes losses and contamination.